Lets start with this. How do you often find yourself using TP on Nasus? Defensively to get back to lane? (Excluding 1st use) or Offensively where you are TPing to a ward and ganking say bot lane. If you use it just as a took to get back to lane everytime, you may need to think of a Sum Swap for awhile. Ghost, how many times do you use it to chase down a kill or to get away? Ask yourself when you die, if I had Flash here over Ghost would of I gotten away? Now if you start to try not bringing TP, for whatever reason, Ghost and Flash would both be acceptable.
Your build Tiamat which isn't Ideal on Nasus, Tri-Force or IBG and straight tank, Death's Dance is also kind of a fun item on Nasus. Starek's would also be acceptable but try not to build more then 2 Offensive items, 3 Tanky (4 Ideal) and either Ninja Tabis, Merc Treads or Swifties for boots.
Pay attention to your Warding totem, if you see two wards or your pushing up, try to ward more. The more wards you use the better off you'll be.
On your support games, how often do you use your activated items (Locket, Redemption, Mikalis), try to make sure your mentally telling yourself to use them all at opportune times.
I'd also work on farming better, knowing when to farm, when to attack or when to farm when to roam is vital knowledge. Your last 10 I looked at you had some decent 5-6 cs/min but you also had some awful 2-3 cs/min. You should strive for around 6.5-7+/min I think that's one of the biggest things players have to learn. Controlling Waves, Map awareness, Warding and Overall skill mechanics. Keep practing you'll get better and you'll climb out.
